The correct answer is: C. x-axis
The x-axis is the horizontal axis on a graph, and the y-axis is the vertical axis. In a cost function graph, the x-axis typically represents the number of machine hours, and the y-axis represents the cost.
The unit axis is the axis that represents the units of measurement for the data. In a cost function graph, the unit axis would typically represent the units of measurement for the cost, such as dollars or euros.
The term axis is the axis that represents the different terms in the cost function. In a cost function graph, the term axis would typically represent the different types of costs, such as fixed costs, variable costs, and semi-variable costs.
I hope this helps! Let me know if you have any other questions.